Custom content types may have relationships to each other or other content types.
For instance, a person may have an image, or an employee may have a boss, or belong
to a department. These relationships must be defined with a specific relationship type, then used
in the custom content with an input type ContentSelector. The relationship type definition
is an XML file. It must be placed in the folder, cms/relationship-types/[name] and
be named relationship-type.xml. Here is an example of a relationship-type: